Cursors is a React mixin that is inspired by David Nolen's Om. This is a
much lighter implementation that is focused on Nolen's idea of a single global
state and using cursors to create smaller local states within a single shared
data structure.

Cursors leverages the Immutability Helpers provided by React Add-ons. By
avoiding mutation, tasks like undo/redo become trivial, reasoning about problems
becomes easier, and bugs are easier to avoid.

Returns a new cursor with its path set to key's path concatenated with
path. key and path should both be a string or number. Use an array of
strings and/or numbers for path if your path goes more than one level
deep.